Cleaning process for electronic and electrical assemblies

ABSTRACT

The invention relates to the use of diacetone alcohol in water-miscible cleaning agents free from halogenated hydrocarbons for cleaning electronic and electrical assemblies. Diacetone alcohol can be used to particularly good effect in the removal of fluxes because, in addition to rosin residues, the so-called &#34;white residues&#34; are also removed.

FIELD OF THE INVENTION STATEMENT OF RELATED ART

This invention relates to water-miscible cleaning agents free fromhalogenated hydrocarbons for cleaning electronic and electricalassemblies and to the use of the cleaning agent for the removal of fluxfrom circuit boards after soldering.

Basically, fluxes based on rosin are made up of products belonging totwo different chemical groups, namely relatively apolar resin-likecompounds, for example rosin, and polar compounds, so-called activators.In accordance with the laws of chemistry, the chemical compoundsdissolve best in solvents of the same type, i.e. in apolar or polarcleaning agents. Particular problems arise in the prior art when thecarboxylic acids used as activators are present in the rosin inconcentrations of up to 35% and, in some cases, are insoluble influorinated hydrocarbons or chlorinated hydrocarbons and only sparinglysoluble in alcohols. This often leads to dissolution of the rosin and toexposure of white residues of the activators which, freed from theinsulating effect of the rosin, can become conductive and corrosiveunder the effect of atmospheric moisture.

Until very recently, fluorinated hydrocarbons were mainly used forcleaning electronic and electrical assemblies, more particularly forremoving rosin-containing fluxes from circuit boards after soldering. Inview of the problem of destruction of the ozone layer bychlorofluorocarbons, attempts have recently been made to developalternative cleaning agents.

DE-A 23 16 720 describes a cleaning agent for removing rosin-containingresidues from electrical equipment which is said largely to eliminaterosin residues and activators, if any, but which on the other hand doesnot affect the insulating materials and metals normally used inelectrical circuits. The cleaning solution contains in particular apolar, higher alcohol, more particularly butyl glycol or n-butanol, apolar lower alcohol, more particularly methanol or isopropanol, inaddition to an apolar hydrocarbon, more particularlytrichlorotrifluoroethane or hexane, optionally in the presence of anonionic wetting agent. Firstly, a water-immiscible cleaning agent isobtained through the use of the polar hydrocarbon. Secondly, thechlorofluorocarbon used should be avoided on account of itsozone-destroying effect. In addition, cleaning agents of the type inquestion have extremely low flash points without the chlorofluorocarbonsso that their practical application requires appropriate explosion-proofequipment.

A cleaning agent containing non-halogenated hydrocarbons, a fluorinatedalcohol and, optionally, other polar organic solvents in addition tohalogenated hydrocarbons is known from EP-A-0 120 319. In view of thepresence of halogenated hydrocarbons, this cleaning agent does notsatisfy present-day requirements.

EP-A-0 416 763 describes the cleaning of electronic circuit boards withpreparations containing terpenes or other non-halogenated solvents.Unfortunately, these cleaning agents have an extremely low flash pointso that appropriate anti-explosion measures have to be taken for theirpractical application.

EP-A-0 419 089 describes a cleaning agent containing dimethylcyclooctadiene.

The use of diacetone alcohol for dissolving rosin and many natural andsynthetic resins is known from Rompps Chemie-Lexikon [Title in English:Rompps Chemical Dictionary], 9th Edition, Vol. 2 (1990), page 923,keyword "diacetone alcohol".

Water-tree solvent mixtures consisting of chlorofluorocarbons andketones are known from Chemical Abstracts, Vol. 114 (1991), page 139,Abstract 114:84372 g from JP-A90/202998. In addition to thechlorofluorocarbons mentioned, diacetone alcohol inter alia is used forremoving fluxes.

DESCRIPTION OF THE INVENTION Object of the Invention

By contrast, the problem addressed by the present invention was toprovide a water-miscible preparation free from halogenareal hydrocarbonsfor cleaning electronic and electrical assemblies which would be capableof safely removing the polar and apolar substances present in thefluxes, particularly those based on rosin, without the use ofchlorofluorocarbons or chlorinated hydrocarbons. In addition, thepreparation would have a flash point above 60° C. which could be furtherincreased by addition of water, depending on the particular application.A further requirement to be satisfied was universal useability inimmersion/flood systems, ultrasonic baths and spray systems.

The cleaning agent according to the invention would also have to becompletely removable by rinsing with water after cleaning.

SUMMARY OF THE INVENTION

It has now been found that water-miscible cleaning agents free fromhalogenated hydrocarbons based on diacetone alcohol are capable ofsatisfying all the requirements mentioned above. In a first embodiment,therefore, the present invention relates to water-miscible preparationsfree from halogenated hydrocarbons and containing diacetone alcohol forcleaning electronic and electrical assemblies. According to theinvention, the cleaning agents contain 30 to 90% by weight of diacetonealcohol, based on the cleaning agent. It has also been found that, inaddition to the good dissolving effect of diacetone alcohol on rosinknown per se, the so-called "white residues" emanating from theactivators could also be removed substantially completely with diacetonealcohol.

DESCRIPTION OF PREFERRED EMBODIMENTS

It has also been found that diacetone alcohol need not necessarily beused in an anhydrous medium for the purposes of the invention. Instead,the diacetone alcohol may also be used in the form of an aqueoussolution. Aqueous solutions preferably contain at least 30% by weight ofdiacetone alcohol, based on the cleaning agent. A diacetone alcoholcontent, based on the cleaning agent, of 30 to 80% by weight isparticularly preferred, a content of 50 to 70% by weight being mostparticularly preferred.

In addition, cleaning agents according to the invention may containother additives. For example, surfactants may be added to reduce thesurface tension. Surfactants suitable for this purpose are, for example,known nonionic and anionic surfactants. Examples of suitable nonionicsurfactants are adducts of ethylene oxide and/or propylene oxide withfatty alcohols, fatty amines, fatty acids or alkyl phenols. Examples ofsuitable anionic surfactants are alkyl benzenesulfonates,alkanesulfates, alkanesulfonates, fatty alkyl ether sulfates orα-sulfofatty acid esters. The cleaning agent may contain up to 10% byweight and, more particularly, from 1 to 5% by weight of thesesurfactants used for the stated purpose.

Corrosion inhibitors for inhibiting corrosion on the metallic surface ofthe article to be cleaned are known in the prior art and, as such, mayalso be used for the purposes of the present invention. For example,copper corrosion inhibitors, such as benzotriazole and/or tolyltriazole, may be used. A suitable concentration range for thesecorrosion inhibitor additives is from 0.01 to 5% by weight and, moreparticularly, from 0.01 to 1% by weight.

In another embodiment, preferred cleaning agents according to theinvention contain water-soluble organic nitrogen bases, moreparticularly triethanolamine, for the accelerated separation of acidicconstituents from the flux resin and, more particularly, the activatorsused. The nitrogen bases are preferably used in a quantity of 1 to 10%by weight and, more preferably, in a quantity of 2 to 5% by weight,based on the cleaning agent.

The cleaning agents according to the invention may also containwater-soluble organic solvents containing one or more polar groups, moreparticularly those containing an alcohol group and having a boilingpoint below 300° C., i.e. linear alcohols, such as butanol, hexylalcohol, decanol, etc. and branched isomers thereof, those containing analicyclic alcohol group, for example embodied in cyclohexanol, thosecontaining several alcohol groups, more particularly glycols, forexample embodied in hexylene glycol, those containing an ether group,glycol ethers, for example embodied in ethylene glycol monobutyl ether,ethylene glycol dibutyl ether, etc., and those containing an estergroup, such as glycolic acid ester or glycol ester, for example ethylglycol acetate. In addition, lactones or lactams, more particularlyN-methyl-2-pyrrolidone, may be present in the cleaning agents accordingto the invention. A key criterion in the choice of these so-calledcleaning boosters is the flash point of the cleaning agent obtainedwhich should not be lowered to values below 60° C. through the additionof these cleaning boosters. These water-soluble solvent cleaningboosters are preferably present in the cleaning agents according to theinvention in a quantity of up to 30% by weight and, more preferably, ina quantity of 5 to 20% by weight, based on the cleaning agent.

In another embodiment of the present invention, the cleaning agents maycontain up to 5% by weight and, more particularly, 1 to 2% by weightalkali metal phosphates, based on the cleaning agents, as builders. Inthe context of the invention, alkali metal phosphates are understood tobe alkali metal orthophosphates, pyrophosphates and triphosphates (alsoreferred to as "tripolyphosphates"). Of these, however, thetriphosphates are preferred for the purposes of the invention, potassiumtriphosphate or sodium triphosphate being particularly preferred.

The percentages by weight mentioned above for the individual componentsshould always be understood to add to 100% by weight.

In the cleaning of printed circuit boards using the cleaning agentaccording to the invention, the circuit boards are immersed in thecleaning agent which is kept at a temperature between room temperatureand about 100° C. and, more particularly, at a temperature of 30° to 70°C. If desired, immersion may be accompanied by ultrasonication,stirring, vibration, blowing through of air, liquid circulation.Alternatively, cleaning may be carried out by spraying, immersion,brushing, flooding and combinations thereof.

After cleaning as described above, the circuit boards may either beremoved (from the cleaning bath) and simply dried or, alternatively, maybe cleaned once more with the same cleaner (bath) or with conventionalhalogen-containing solvents, such as chlorofluorocarbons orchlorine-containing cleaners or cleaners of the alcohol type. Thequantities of halogen-containing cleaners used for this purpose aredistinctly smaller than the quantities in which the halogen-containingcleaners are used in the first cleaning step.

After cleaning, the circuit boards are preferably rinsed with water. Itis of particular advantage to use water when either a considerablequantity of water-soluble substances remains behind on the circuitboards or when fine dust-like residues are present thereon in relativelylarge quantities. The water used for rinsing is not contaminated withhalogenated hydrocarbons, as is the case with corresponding cleaningagents of the prior art, but instead contains only biodegradablesubstances.

In addition, the printed circuit boards may be dried after the treatmentdescribed above, for example at elevated temperature in a drying cabinetor in a suitable dryer, by a stream of hot air or by washing with areadily volatile solvent, such as ethanol and/or isopropanol, whichforms an azeotrope with water.

Accordingly, the present invention also relates to a process forcleaning electronic and electrical assemblies using water-misciblecleaning agents free from halogenated hydrocarbons of the type definedabove. More particularly, the invention relates to a process for theremoval of fluxes after soldering.

The invention is illustrated by the following Examples.

EXAMPLE 1

A rosin-based flux is applied to a circuit board and dried at 150° to200° C. The circuits are then soldered by hand using a commerciallyavailable solder.

The board thus treated was ultrasonicated for 2 minutes at 55° C. in anaqueous solution containing 60% by weight of diacetone alcohol, 12% byweight N-methyl-2-pyrrolidone, 2% by weight of a nonionic surfactant(C₁₃ alkyl polyethylene glycol ether containing 5 ethylene oxide units)and 1% by weight of an anionic surfactant (C₁₃₋₁₇ alkylsulfonic acid, Nasalt). The circuit board was then rinsed with water and dried in adrying cabinet.

After this treatment, the surface was microscopically clean and nocoatings (white deposits) were in evidence, even on the circuits.

EXAMPLE 2

Another circuit board was pretreated as described in Example 1 and thencleaned by spraying for 3 minutes by spraying at 55° C. (sprayingpressure 4 bar) with an aqueous solution of 60% by weight of diacetonealcohol, 12% by weight of N-methyl-2-pyrrolidone, 2% by weight oftriethanolamine, 2% by weight of a nonionic surfactant (C₁₃ alkylpolyethylene glycol ether containing 5 ethylene oxide units) and 1% byweight of an anionic surfactant (C₁₃₋₁₇ alkylsulfonic acid, Na salt).

After rinsing with water and drying, the surfaces showed no residues ofrosin under a microscope and even the circuits were bright and free fromany deposits.

EXAMPLE 3

After reflow soldering, a circuit board provided with solder paste andequipped with SMD units was ultrasonicated for 3 minutes at 45° C. in asolvent mixture containing 86% by weight of diacetone alcohol, 9% byweight of N-methyl-2-pyrrolidone, 2% by weight of triethanolamine, 2% byweight of C₁₃ alkyl polyethylene glycol ether containing 5 ethyleneoxide units and 1% by weight of C₁₃₋₁₇ alkylsulfonic acid Na salt.

The circuit board was then rinsed with water and dried in a dryingcabinet. After this treatment, the surface of the circuit board wascompletely clean and did not show any residues of the solder paste, evenin the vicinity of the contact feet and in the narrow gaps beneath theSMD units.

The invention claimed is:
 1. A process for cleaning resin flux fromelectronic and electrical assemblies comprising contacting saidelectronic and electrical assemblies with a water-miscible cleaningagent free from halogenated hydrocarbons and containing diacetonealcohol in quantities of 30 to 90% by weight, based on the weight of thecleaning agent.
 2. A process as claimed in claim 1, wherein the cleaningagent contains diacetone alcohol in quantities of 30 to 80% by weight,based on the cleaning agent, in an aqueous solution.
 3. A process asclaimed in claim 2, wherein the cleaning agent contains diacetonealcohol in quantities of 50 to 70% by weight in an aqueous solution. 4.A process as claimed in claim 3, wherein the cleaning agent containsanionic surfactants, nonionic surfactants, or both in a quantity of upto 10% by weight, based on the cleaning agent.
 5. A process as claimedin claim 4, wherein the cleaning agent contains anionic surfactants,nonionic surfactants, or both in quantities of 1 to 5% by weight.
 6. Aprocess as claimed in claim 5, wherein the cleaning agent containswater-soluble organic nitrogen bases in quantities of 1 to 10% byweight, based on the cleaning agent.
 7. A process as claimed in claim 6,wherein the cleaning agent contains triethanolamine in quantities of 2to 5% by weight.
 8. A process as claimed in claim 7, wherein thecleaning agent contains water-soluble solvents selected from alcohols,glycols, glycol ethers, glycol esters, glycolic acid esters, lactonesand lactams in a quantity of up to 30% by weight, based on the cleaningagent.
 9. A process as claimed in claim 8, wherein the cleaning agentcontains N-methyl-2-pyrrolidone in quantities of 5 to 20% by weight. 10.A process as claimed in claim 9, wherein the cleaning agent containsinorganic builders selected from the group consisting of alkali metalphosphates in a quantity of at most 5% by weight, based on the cleaningagent.
 11. A process as claimed in claim 10, wherein the cleaning agentcontains inorganic builders in quantities of 1 to 2% by weight.
 12. Aprocess as claimed in claim 11, wherein the cleaning agent containscorrosion inhibitors in quantities of 0.01 to 5% by weight, based on thecleaning agent.
 13. A process as claimed in claim 12, wherein thecleaning agent contains corrosion inhibitors in quantities of 0.01 to 1%by weight.
 14. A process as claimed in claim 1, wherein the cleaningagent is used to remove fluxes after soldering.
 15. A process as claimedin claim 14, wherein the cleaning process is assisted byultrasonication, stirring, vibration, blowing through of air, liquidcirculation of the cleaning agent or by spraying, immersion, brushing,flooding or combinations thereof.
 16. A process as claimed in claim 1,wherein the cleaning agent contains anionic surfactants, nonionicsurfactants, or both in a quantity of up to 10% by weight, based on thecleaning agent.
 17. A process as claimed in claim 1, wherein thecleaning agent contains water-soluble organic nitrogen bases inquantities of 1 to 10% by weight, based on the cleaning agent.
 18. Aprocess as claimed in claim 1, wherein the cleaning agent containsinorganic builders selected from the group consisting of alkali metalphosphates in a quantity of at most 5% by weight, based on the cleaningagent.
 19. A process as claimed in claim 1, wherein the cleaning agentcontains corrosion inhibitors in quantities of 0.01 to 5% by weight,based on the cleaning agent.